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Peculiarities of ferromagnetic resonance in HgCr2Se4
PECULIARITIES OF FERROMAGNETIC RESONANCE IN HgCrzSe4 K. G. N i k i f o r o v , * L. Ya. Pasenko,* S, I. R a d a u t s a n , * V. E. T e z l e v a n , * A. G. G u r e v i c h t and L. M. E m i r y a n t *institute of Applied Physics, Academy of Sciences of the Moldavian SSR,
Kishinev, 277028, USSR **loffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad, 194021, USSR
ABSTRACT The low temperature coexistence of chromium ions with minority valency has been studied in the HgCr2Se 4 crystals by ferromagnetic resonance method.
KEYWORDS Magnetic semiconductor HgCr2Se4; minority valency ion; structural defect; thermal treatment; ferromagnetic resonance.
INTRODUCTION The presence of two high vapour pressure components in magnetic semiconductor HgCr2Se 4 leads to a strong dependence of physical properties upon the conditions of the crystal growth (Emiryan and others, 1981; Filatov and others, 1983; Nikiforov, Emiryan and Pasenko, 1983) and annealing (Ferreira and others, 1983; Goldstein, Gibart and Selmi, 1978; Oudet and others, 1983). Our paper maintains some experimental results concerning the influence of various thermal treatments on the ferromagnetic resonance (F~R) in HgCr2Se 4.
EXPERIMENTS AND RESULTS The undoped HgCr2Se 4 single crystals were grown by the chemical vapour transport technique (transport agent - chlorine from AiC13, temperatures of source-zone 670 °C and growth-zone 625 °C). Carefully polished spherical samples have been oriented by X-ray diffraction in a ~ I ~ direction.
379
380
K.G. Nikiforov
etaL
In order to change the quantity of structural defects some spheres were heated for 72 hours. After such treatments these samples were polished and oriented again. FNR parameters - resonance field Hre s and line width 2AH have been measured at a frequency of ~ 9 GHz at the temperature 4.2-150 K. The Hres(@) and 2~H(@) dependences at 4.2 K (where @ is the angle between magnetic field and (I0@ axis in ~ 1 4 plane) are shown in Fig. I. They have a complex character with maxima along
~11)
and
000~
axes.
3100 I
®
2
29o01
IjX.2x. , -
I
<100>
J
-i
(11 I>
(110> 300
j
200
OJ
100 |
-30
O
|
0
30 ~
I
60
I
90
0
Pig. I. Hre s (I-3) and 2AH (I'-3') angular dependences for HgCr2Se 4 before (1,1') and after annealing in Se atmosphere (2,2') and in vacuum (3,3'). Unexpectedly the annealing of HgCr2Se 4 in vacuum (1.5-10 -2 Pa, 300 °C) strongly decreases 2AH in all directions and displaces the Hres(@) dependence by ~I00 0e in the region of high magnetic fields. The treatment in Se vapour (8-102 Pa, 400 °C) acted in the same manner. The standard calculations of the magnetocrystalline anisotropy constants show that our experimental results may be described at least by two first constants
(K I z O
and K 2 ~ 0 )
moreover K2~IKII
The resonance field temperature dependences
in all the cases. (Fig. 2) for our samples have
Ferromagnetic resonance in HgCr2Se4
381
I 3300
~32oo ~100
3000
|
0
! 50
!
100
T, K Fig. 2. Hre s versus temperature for HgCr2Se 4 before (I-3) and after vacuum annealing (I'-3') in various directions: 1,1'-~00); 2 , 2 ' - ~ I ~ 3,3'-~111~. a sharp displacement at helium temperatures. 2AH(T) are characterized by a powerful maxima at 13-16 K for (100) and (111) directions. When the temperature approaches the Curie point (110 K) the Hre s anisotropy decreases and Hre s value strives for Ho~-3160 Oe which corresponds to the g-factor 2.06. The annealing of HgCr2Se 4 samples in Hg vapour (2.105 Pa, 400 °C) leads to a positive magnetic anisotropy (the maxima only in (111~ directions) and strong 2AH increase (Fig. 3). The Hre s anisotropy does not exceed 150 Oe and the resonance line is of no Lorentz shape and it has the sharp peak near H o N 3 kOe. After the maximum 2AH(T) (near ~15 K) it strongly decreases with the temperature increase up to 140 K.
DISCUSSION The maxima of the FMR parameters angular dependences are caused by the near-crossing of the lowest orbital levels of the "impurity" magnetic ions with strong spin-orbital couple, such as chromium ions with minority valency in the magnetic semiconductors CdCr2Se4-type (Gurevich, 1973~ Gurevich and others, 1980). The simultaneous presence of FMR parameters maxima in (100) and (111~ directions testifies the coexistence of minority valency Cr 4+ and
382
K . G . Nikiforov et al.
°ooI ,oo 0
\ 5O
tO0
T, K Fig. 3. 2AH versus temperature for HgCr2Se 4 before and after Hg annealing (2) for the (111) direction.
(I)
Cr 2+ ions (Nikiforov and others, 1983) in HgCr2Se 4 caused by the tetrahedral cation and anion vacancies respectively. In our opinion, the annealing of HgCr2Se 4 in vacuum or in Se atmosphere first of all places the electron change reactions between chromium ions in contrast with the change of vacancy concentration in other magnetic semiconductors (Gurevich and others, 1980). This results in simultaneous decrease of the Cr 2+ and Cr 4+ ion concentrations. The same peculiarities of the FMR temperature dependences are characteristic of the slow relaxation (Gurevich, 1973). Such processes lead to the negative Hre s displacement, proportional to 2AH (i.e. to the Cr 2+ and Cr 4+ ion quantity in our case). The great F~R parameters maxima in
000)
may be explained by the Hg vapour
predominance over the vapour pressure of other components of HgCr2Se4, which results in the Hg vacancy prevalence. The peaks in ~111) directions (formed by Cr 2+ ions) can be due to the Se vacancies presence and the chlorine atom entrance into the anion sites of HgCr2Se 4 spinel structure
(Ametani,
1976).
The annealing at high vapour pressure "heals" the Hg vacancies but it does not prevent from the Cr 2+ ion formation (due to the Se vacancies). This treatment evokes a sharp conductivity increase of the annealing samples at helium temperatures (Goldstein, Gibart and Selmi, 1978). Than the observed 2AH increase can be due to the skin-effect and/or spin-electron relaxation deposit
(Turov, 1961).
Thus our results of the F~R studies in HgCr2Se 4 indicate that the interactions
Ferromagnetic resonance in HgCr2Se4
383
between structural defects and chromium ions with minority valency have more complex character than in other ternary magnetic semiconductors.
REFERENCES
Ametani, K. (1976). Estimation of chlorine content in single crystals of chromium chalcogenide spinels by atomic absorption spectrophotometry. Bull, Chem, Soc~ Jpn,, 47, 242-243. Emiryan, L. M., A° G. Gurevich, A. S. Shukyurov, and V. N. Berzhanskii (1981). Ferromagnetic resonance peculiarities in HgCr2Se 4 magnetic semiconductor. Fiz ~ Tverd t Tela, 23, 2916-2923. Perreira, J. M., M. D. Coutinho-Pilho, S. M. Rezende, and P. Gibart (1983). P~R studies in the ferromagnetic semiconductors CdCr2Se 4 and HgCr2Se 4. J. Magn. Magn. Mater~, ~I-~4, 672-674. Filatov A. V., V. A. Levshin, V° N. Novotortsev, and I. S. Kovaleva (1983). Nonstoichiometry influence on conductivity and Hall effect in ferromagnetic semiconductor Hg1_xZnxCr2Se 4. Proc. All-Union Conf. "Ternary semiconductors and their application", Kishinev, p. 103. Goldstein, L., P. Gibart, and A. Selmi (1978). Transport properties of the ferromagnetic semiconductor HgCr2Se 4. J~ Appl~ Phys., 49, 1474-1476. Gurevich, A. G. (1973). Magnetic resonance in ferrites and antiferrgma~netics. Nauka, Moscow. Gurevich, A. G., L. M. Emiryan, K. G. Nikiforov, and S. I. Radautsan (1980). Chromium ions with variable valence in magnetic semiconductor spinels. Dig. techn, progr, 4th Int. Conf. Ternary and Multinar 2 Compounds, Tokyo, pp. 1 1 1 - 1 1 2 . Nikiforov, K. G., L. M° Emiryan, and L. Ya. Pasenko (1983). Structural vacancy influence on conductivity and ferromagnetic resonance in HgCr2Se 4. Proc. 17th All-Union Conf. Phys. Magn. Phenomena, Tula, pt. I, pp. 68-69. Nikiforov, K. G., S. I. Radautsan, V. E. Tezlevan, A. G. Gurevich, and L. M. Emiryan (1983). Coexistence of minority valency Cr 2+ and Cr 4+ ions in the ternary magnetic semiconductor CdCr2S 4. Nuovo Cim~ D, ~, 1891-1895. Oudet, Ho, P. Gibart, M. Porte, T. Merceron, and G. Villers (1983). ~agnetocrystalline anisotropy of the spinel magnetic semiconductor HgCr2Se 4. Ferrites: Proc. Int. Conf, 1980 Japan I Tokyo, Dordrecht, pp. 909-913. Turov, E. A. (1961). Ferromagnetic resonance peculiarities in metals. In S. V. Vonsovskii (Ed.), Ferromagnetic resonance, GIFML, Moscow, pp. 170-214.
Kishinev, 277028, USSR **loffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad, 194021, USSR
ABSTRACT The low temperature coexistence of chromium ions with minority valency has been studied in the HgCr2Se 4 crystals by ferromagnetic resonance method.
KEYWORDS Magnetic semiconductor HgCr2Se4; minority valency ion; structural defect; thermal treatment; ferromagnetic resonance.
INTRODUCTION The presence of two high vapour pressure components in magnetic semiconductor HgCr2Se 4 leads to a strong dependence of physical properties upon the conditions of the crystal growth (Emiryan and others, 1981; Filatov and others, 1983; Nikiforov, Emiryan and Pasenko, 1983) and annealing (Ferreira and others, 1983; Goldstein, Gibart and Selmi, 1978; Oudet and others, 1983). Our paper maintains some experimental results concerning the influence of various thermal treatments on the ferromagnetic resonance (F~R) in HgCr2Se 4.
EXPERIMENTS AND RESULTS The undoped HgCr2Se 4 single crystals were grown by the chemical vapour transport technique (transport agent - chlorine from AiC13, temperatures of source-zone 670 °C and growth-zone 625 °C). Carefully polished spherical samples have been oriented by X-ray diffraction in a ~ I ~ direction.
379
380
K.G. Nikiforov
etaL
In order to change the quantity of structural defects some spheres were heated for 72 hours. After such treatments these samples were polished and oriented again. FNR parameters - resonance field Hre s and line width 2AH have been measured at a frequency of ~ 9 GHz at the temperature 4.2-150 K. The Hres(@) and 2~H(@) dependences at 4.2 K (where @ is the angle between magnetic field and (I0@ axis in ~ 1 4 plane) are shown in Fig. I. They have a complex character with maxima along
~11)
and
000~
axes.
3100 I
®
2
29o01
IjX.2x. , -
I
<100>
J
-i
(11 I>
(110> 300
j
200
OJ
100 |
-30
O
|
0
30 ~
I
60
I
90
0
Pig. I. Hre s (I-3) and 2AH (I'-3') angular dependences for HgCr2Se 4 before (1,1') and after annealing in Se atmosphere (2,2') and in vacuum (3,3'). Unexpectedly the annealing of HgCr2Se 4 in vacuum (1.5-10 -2 Pa, 300 °C) strongly decreases 2AH in all directions and displaces the Hres(@) dependence by ~I00 0e in the region of high magnetic fields. The treatment in Se vapour (8-102 Pa, 400 °C) acted in the same manner. The standard calculations of the magnetocrystalline anisotropy constants show that our experimental results may be described at least by two first constants
(K I z O
and K 2 ~ 0 )
moreover K2~IKII
The resonance field temperature dependences
in all the cases. (Fig. 2) for our samples have
Ferromagnetic resonance in HgCr2Se4
381
I 3300
~32oo ~100
3000
|
0
! 50
!
100
T, K Fig. 2. Hre s versus temperature for HgCr2Se 4 before (I-3) and after vacuum annealing (I'-3') in various directions: 1,1'-~00); 2 , 2 ' - ~ I ~ 3,3'-~111~. a sharp displacement at helium temperatures. 2AH(T) are characterized by a powerful maxima at 13-16 K for (100) and (111) directions. When the temperature approaches the Curie point (110 K) the Hre s anisotropy decreases and Hre s value strives for Ho~-3160 Oe which corresponds to the g-factor 2.06. The annealing of HgCr2Se 4 samples in Hg vapour (2.105 Pa, 400 °C) leads to a positive magnetic anisotropy (the maxima only in (111~ directions) and strong 2AH increase (Fig. 3). The Hre s anisotropy does not exceed 150 Oe and the resonance line is of no Lorentz shape and it has the sharp peak near H o N 3 kOe. After the maximum 2AH(T) (near ~15 K) it strongly decreases with the temperature increase up to 140 K.
DISCUSSION The maxima of the FMR parameters angular dependences are caused by the near-crossing of the lowest orbital levels of the "impurity" magnetic ions with strong spin-orbital couple, such as chromium ions with minority valency in the magnetic semiconductors CdCr2Se4-type (Gurevich, 1973~ Gurevich and others, 1980). The simultaneous presence of FMR parameters maxima in (100) and (111~ directions testifies the coexistence of minority valency Cr 4+ and
382
K . G . Nikiforov et al.
°ooI ,oo 0
\ 5O
tO0
T, K Fig. 3. 2AH versus temperature for HgCr2Se 4 before and after Hg annealing (2) for the (111) direction.
(I)
Cr 2+ ions (Nikiforov and others, 1983) in HgCr2Se 4 caused by the tetrahedral cation and anion vacancies respectively. In our opinion, the annealing of HgCr2Se 4 in vacuum or in Se atmosphere first of all places the electron change reactions between chromium ions in contrast with the change of vacancy concentration in other magnetic semiconductors (Gurevich and others, 1980). This results in simultaneous decrease of the Cr 2+ and Cr 4+ ion concentrations. The same peculiarities of the FMR temperature dependences are characteristic of the slow relaxation (Gurevich, 1973). Such processes lead to the negative Hre s displacement, proportional to 2AH (i.e. to the Cr 2+ and Cr 4+ ion quantity in our case). The great F~R parameters maxima in
000)
may be explained by the Hg vapour
predominance over the vapour pressure of other components of HgCr2Se4, which results in the Hg vacancy prevalence. The peaks in ~111) directions (formed by Cr 2+ ions) can be due to the Se vacancies presence and the chlorine atom entrance into the anion sites of HgCr2Se 4 spinel structure
(Ametani,
1976).
The annealing at high vapour pressure "heals" the Hg vacancies but it does not prevent from the Cr 2+ ion formation (due to the Se vacancies). This treatment evokes a sharp conductivity increase of the annealing samples at helium temperatures (Goldstein, Gibart and Selmi, 1978). Than the observed 2AH increase can be due to the skin-effect and/or spin-electron relaxation deposit
(Turov, 1961).
Thus our results of the F~R studies in HgCr2Se 4 indicate that the interactions
Ferromagnetic resonance in HgCr2Se4
383
between structural defects and chromium ions with minority valency have more complex character than in other ternary magnetic semiconductors.
REFERENCES
Ametani, K. (1976). Estimation of chlorine content in single crystals of chromium chalcogenide spinels by atomic absorption spectrophotometry. Bull, Chem, Soc~ Jpn,, 47, 242-243. Emiryan, L. M., A° G. Gurevich, A. S. Shukyurov, and V. N. Berzhanskii (1981). Ferromagnetic resonance peculiarities in HgCr2Se 4 magnetic semiconductor. Fiz ~ Tverd t Tela, 23, 2916-2923. Perreira, J. M., M. D. Coutinho-Pilho, S. M. Rezende, and P. Gibart (1983). P~R studies in the ferromagnetic semiconductors CdCr2Se 4 and HgCr2Se 4. J. Magn. Magn. Mater~, ~I-~4, 672-674. Filatov A. V., V. A. Levshin, V° N. Novotortsev, and I. S. Kovaleva (1983). Nonstoichiometry influence on conductivity and Hall effect in ferromagnetic semiconductor Hg1_xZnxCr2Se 4. Proc. All-Union Conf. "Ternary semiconductors and their application", Kishinev, p. 103. Goldstein, L., P. Gibart, and A. Selmi (1978). Transport properties of the ferromagnetic semiconductor HgCr2Se 4. J~ Appl~ Phys., 49, 1474-1476. Gurevich, A. G. (1973). Magnetic resonance in ferrites and antiferrgma~netics. Nauka, Moscow. Gurevich, A. G., L. M. Emiryan, K. G. Nikiforov, and S. I. Radautsan (1980). Chromium ions with variable valence in magnetic semiconductor spinels. Dig. techn, progr, 4th Int. Conf. Ternary and Multinar 2 Compounds, Tokyo, pp. 1 1 1 - 1 1 2 . Nikiforov, K. G., L. M° Emiryan, and L. Ya. Pasenko (1983). Structural vacancy influence on conductivity and ferromagnetic resonance in HgCr2Se 4. Proc. 17th All-Union Conf. Phys. Magn. Phenomena, Tula, pt. I, pp. 68-69. Nikiforov, K. G., S. I. Radautsan, V. E. Tezlevan, A. G. Gurevich, and L. M. Emiryan (1983). Coexistence of minority valency Cr 2+ and Cr 4+ ions in the ternary magnetic semiconductor CdCr2S 4. Nuovo Cim~ D, ~, 1891-1895. Oudet, Ho, P. Gibart, M. Porte, T. Merceron, and G. Villers (1983). ~agnetocrystalline anisotropy of the spinel magnetic semiconductor HgCr2Se 4. Ferrites: Proc. Int. Conf, 1980 Japan I Tokyo, Dordrecht, pp. 909-913. Turov, E. A. (1961). Ferromagnetic resonance peculiarities in metals. In S. V. Vonsovskii (Ed.), Ferromagnetic resonance, GIFML, Moscow, pp. 170-214.